1. Technological Field of the Invention
The invention relates to a neutron absorbent composite material and a method of manufacturing this material.
Neutron absorbent materials are neutron absorbers. They find application for example, in the manufacture of control rods which are used for the control of the reactivity of nuclear reactors, notably for the control of pressurized water nuclear reactors (PWRs).
In effect, when fission of a heavy nucleus occurs in the core of a nuclear reactor, neutrons are emitted in the free state. The neutron is a projectile capable of causing fission of heavy nuclei. If among the emitted neutrons, certain of them have the opportunity of colliding with a fissile nucleus and causing its fission, they will in their turn generate descendants which themselves can cause the fission of another nucleus and give rise to further generations in a chain reaction. It is therefore clearly important to control the quantity of free neutrons formed so as to prevent the fission reaction accelerating and to keep the fission in a critical state, that is to say, in equilibrium.
Hence, control rods comprising neutron absorbent materials are movable rods mounted in the core of nuclear reactors in such a way that they are able to slide between the fuel assemblies or within the network formed by an assembly of fuel rods. Control of fission in the core is by inserting or withdrawing these rods from the core of the nuclear reactor by sliding them in and out.
Absorbent materials can be used to maintain the nuclear fission in the critical state, in which case they constitute control rods. They can be used to ensure the rapid halting of the chain reaction, in which case they then constitute safety shut-off rods.
In order to be efficient in the control of nuclear reactors, the neutron absorbent material must meet the following selection criteria it must have a high effective neutron absorption cross section, good mechanical characteristics, good chemical resistance and good dimensional stability at temperature and under irradiation.
In certain cases, the neutron absorbent material must be covered with a sheath, generally made of stainless steel. It must be chemically compatible with this sheath.
In addition, the costs of the raw materials and the cost of manufacturing the neutron absorbent material must be kept reasonable.
2. Prior Art
At the present time, the neutron absorbent materials most widely used in the control rods for pressurized light water nuclear reactors (PWRs) are boron carbide (B4C), and a metal alloy of silver, indium and cadmium (SIC).
These materials have the advantage of having an effective neutron absorption cross section that meets the selection criteria for neutron absorbent materials.
The B4C absorbent material is used in the form of stacks of sintered cylindrical pellets produced from powders.
Although it is highly chemically inert, B4C oxidizes starting from 600xc2x0 C. in the presence of oxygen. This compound is also sensitive to corrosion by the water in the primary PWR medium, notably when it has been irradiated by neutrons. This is one of the reasons why it is inserted into sheaths that are generally made of stainless steel.
In addition, the life of the boron carbide never reaches the theoretical limit fixed by the depletion of the boron because of damage to the material caused by the large quantity of helium and lithium formed by the absorption of neutrons. In effect, under the effect of the temperature, a part of the helium formed diffuses out of the material while the other part accumulates in it, causing swelling and micro-fractures in the material.
The combination of the swelling and the micro-fractures can cause, under strong irradiation, a mechanical interaction between the absorbent material and the steel sheath which can lead to rupture of the sheath, which is itself weakened by the fast neutron irradiation on the one hand and by the diffusion of a certain quantity of boron and carbide from the absorbent material on the other hand.
SIC absorbent materials comprise by mass about 80% silver, 15% indium and 5% cadmium. These SIC materials are used in cylindrical sheaths made of stainless steel since they have poor resistance to corrosion at the operating temperatures of nuclear reactors, in water that may incidentally contain oxygen.
SIC has good physical and chemical properties under irradiation and the modifications to which this material is subject in the course of the neutron absorption are considered to be acceptable for the control rods of present day PWRS. However, the very low melting point of this material and the cost of the silver which it contains are disadvantages that cannot be ignored for the use of this material for the control of nuclear reactors.
The materials B4C and SIC do not therefore meet the selection criteria described previously, to a satisfactory extent.
The precise aim of this invention is to provide a neutron absorbent material which enables one to resolve the problems described above, as well as a method of manufacturing said material.
According to the invention, the neutron absorbent material is a composite material comprising hafnium diboride and hafnium dioxide.
According to the invention, hafnium diboride can represent preferably at least 80% by volume of the material, more preferably about 90% by volume of the material.
According to the invention, hafnium dioxide can represent preferably up to 20% by volume of the material, more preferably up to 10% by volume of the material.
According to the invention, the hafnium diboride can be in the form of particles in the composite material, said particles preferably having a diameter ranging up to about 50 xcexcm.
According to the invention, the hafnium dioxide an be in the form of particles in the composite material, said particles preferably having a diameter ranging up to about 20 xcexcm, more preferably ranging up to about 10 xcexcm.
According to the invention, the composite material comprising hafnium diboride and hafnium dioxide of the invention, can have a density of about 10000 to 11000 kg/m3, preferably about 10550 to 10630 kg/m3, and more preferably about 10590 kg/m3.
The neutron absorbent material conforming to the invention has the advantage of greater resistance to corrosion by the water of the primary medium in the PWR, that is to say that it contains a maximum content of 2500 ppm dissolved boron and 2.5 ppm dissolved lithium, at a temperature of about 345xc2x0 C. and at a pressure of about 155 bars, this being translated as a quasi-zero dissolution of boron in the water.
Another advantage of the material according to the invention is that it keeps its integrity after a corrosion test lasting 1000 hours at a temperature of 345xc2x0 C. and at a pressure of 15.5xc3x97106 Pa in water that is representative of that found in the primary medium of a PWR.
Another advantage of the material according to the invention has been revealed by tests carried out on a pure HfB2 material at a temperature of 345xc2x0 C. and at a pressure of 15.5xc3x97106 Pa in water that is representative of that found in the primary medium of a PWR. These tests have shown fracturing of this material caused by the formation of corrosion pits rich in oxygen called the oxide phase, within the mass of the pellet. In effect, these pits have generated internal stresses because of the density difference between the oxide phase and the boride phase, which have caused fracturing of the pellets.
In the case of the composite material according to the invention, corrosion pits are also formed but they are of much reduced size, since their development has been blocked by the presence of hafnium dioxide which has limited their propagation.
This result translates itself into increased toughness of the composite material of this invention which is greater than that of pure HfB2.
The neutron absorbent composite material according to the invention can be described as comprising a homogeneous matrix of hafnium diboride (HfB2) in which fine particles of hafnium dioxide (HfO2) are dispersed in a homogeneous fashion.
This invention also relates to a method of manufacturing a neutron absorbent material, said neutron absorbent material being a composite material comprising hafnium diboride.
This method comprises steps that consist of, in this order:
adding hafnium dioxide powder to hafnium diboride powder,
mixing the hafnium diboride powder and the hafnium dioxide powder in a way that produces a homogeneous mixture, and
sintering the homogeneous mixture in a way that produces the composite material.
According to the method of the invention, up to 20% by volume of hafnium dioxide, preferably about 10% by volume of hafnium dioxide, can be added, the homogeneous mixture of hafnium diboride and hafnium dioxide powders representing 100% by volume.
According to the method of the invention, the hafnium diboride powder can have a particle size preferably ranging up to about 50 xcexcm.
According to the method of the invention, the hafnium dioxide powder can have a particle size preferably ranging up to about 20 xcexcm, more preferably up to about 10 xcexcm.
According to the method of the invention, the mixture of the hafnium diboride powder and the hafnium dioxide can be produced by any method known to a man skilled in the art to obtain a homogeneous mixture of such powders. Preferably, the mixture of these powders can be produced by application of ultra-sound to a slip comprising said powders dispersed in a dispersion liquid.
The dispersion liquid is preferably a liquid which after the mixing of the powders, may be easily removed, for example, by evaporation. This dispersion liquid may be, for example, an alcohol such as ethanol.
As soon as a homogeneous mixture is obtained, it can be dried, for example by evaporation of the alcohol and then it can be sieved so as to remove any possible aggregates of powder in the mixture.
The homogeneous mixture obtained is then sintered in order to obtain the composite material.
According to the invention, the sintering can be carried out under vacuum.
According to the invention, the mixture can be sintered in any mold suitable for sintering such powders, for example a graphite mold preferably lined by a sheet of graphite.
Lining the mold enables one to avoid diffusion of chemical species from the mixture to the mold and facilitates the subsequent stripping of the composite material from the mold.
The mold can, for example, have a shape suitable for the molding of the material in the form of a pellet, plate, cross, rod and in a general way a shape suitable to constitute control rods for a nuclear reactor.
Sintering of the mixture is carried out under conditions of temperature, pressure and duration that permit appropriate densification of the two materials. It can, for example, be carried out at a temperature of about 1600 to 2100xc2x0 C., preferably at a temperature of about 1900xc2x0 C., under a pressure of 10 to 100 MPa, preferably about 83 MPa, for a period of about 15 to 90 minutes, preferably about 1 hour, for example in a furnace held under a dynamic vacuum.
Following this sintering, or heat treatment, the composite material obtained can be machined, for example, using diamond tipped tools. In effect, the cortical area of the composite material can have fine fissures due to a chemical reaction between the oxide HfO2 present in the material and the mold, for example, a graphite mold. This cortical area can be removed by machining over a thickness of from 500 to 1000 xcexcm, preferably 750 xcexcm.
The method of the invention notably permits a reduction in the sintering temperature of the composite material of about 200xc2x0 C. compared with that for pure hafnium diboride.
Other characteristics and advantages will better become apparent on reading the following example that makes reference to the appended drawings, which is given, it is understood, for information purposes only and which is non-limitative.